The influence of sediment recycling and basement ... · The influence of sediment recycling and...

Pergamon

Geochimica et Cosmochimica Acta, Vol. 59, No. 14. pp. 2919-2940, 1995 Copyright 0 1995 Elsevier Science Ltd Primed in the USA. All rights reserved

COl6-7037/95 $9.50 + .OO

0016-7037(95)00185-9

The influence of sediment recycling and basement composition on evolution of mudrock chemistry in the southwestern United States

R~NADH Cox,‘.* DONALD R. LOWE,* and R. L. CULLERS’

‘Department of Geology, Rand Afrikaans University, Johannesburg 2006, South Africa ‘Department of Geology, Stanford University, CA 94305-2115, USA ‘Department of Geology, Kansas State University, KS 66506, USA

(Received April I, 1994; accepted in revised form April 5, 1995 )

Abstract-This paper reports systematic changes in mudrock composition through time on a single continental cmstal block. The changes reflect both sediment recycling processes and changes through time in the composition of crystalline material being added to the sedimentary system and are related to tectonic evolution as the block matures from a series of accreted arc terranes to a stable craton.

The major and trace element distributions reflect different aspects of the provenance of the mudrocks in this study. Major elements record sediment recycling processes as well as changing proportions of sedimentary and first-cycle source rocks. With the exceptions of KzO (which tends to increase), and SiOz and A&O3 (which show no trend), most major oxides tend to decline in relative abundance in younger mudrocks. Patterns shown by the Index of Compositional Variability ( [ Fe03 + KrO + NaaO + CaO + MgO + MnO + TiOJIAlrO,) and by K20/A1203 indicate that the major oxide trends are due to decreasing proportions of nonclay silicate minerals and a concomitant increase in the proportion of clay minerals, probably due to decreasing input of first cycle detritus coupled with recycling of sedimentary material. Excursions from progressive trends, marked by increases in MgO, K20, and CaO, reflect episodes of large- scale input of nonclay first-cycle minerals from crystalline source rocks due to large-scale basement uplift.

The chemistry of low-solubility trace elements, in contrast, is not sensitive to recycling effects and reflects the composition of first-cycle input. Incompatible elements are progressively enriched relative to compatible elements in younger mudrocks, and values for chondrite normalised rare earth elements also increase. In addition, the Eu anomaly becomes systematically more negative in younger samples. These trends cMnot be explained by diagenetic or weathering processes, and, therefore, indicate that the proportion of fnc- tionated granitic first-cycle detritus being added to the sedimentary system becomes greater with time.

These results confirm the importance of tectonic setting in controlling mudrock chemistry, and also demonstrate that there is a dynamic relationship between the tectonic evolution of a continental block and the composition of its sedimentary mantle.

1. INTRODUCTION

The generalisation is frequently made that mudrocks represent samples of the average continental crust (e.g., All&e and Rousseau, 1984; Taylor and McLennan, 1985)) which is cer- tainly true of muds from the ocean basins, the constituents of which have been transported long distances and thoroughly mixed. Most mudrocks, however, form in more confined basins and from more restricted source rocks and are not glob- ally homogenised: Bhatia and Taylor ( 1981) and Bhatia ( 1985) have shown that the compositions of mudrocks from volcanic arc settings are related to the compositions of their source rocks, and bear little resemblance to average shale compositions.

In studies of global variation in mudrock chemistry (Gar- rels and Mackenzie, 197 1; Ronov and Migdisov, 197 1; Taylor and McLennan, 1985 ), the average mudrock composition for any given time interval is typically computed from a collec- tion of individual mudrock samples, each of which was derived from specific source rock types in specific tectonic set-

* Author to whom correspondence should be addressed.

tings. Information about the controls on mudrock composition on a local scale is, therefore, critical to an understanding of variations in mudrock composition on a global scale. How- ever, with the exception of the work of Bhatia and co-workers (Bhatia and Taylor, 1981; Bhatia, 1983; Bhatia and Crook, 1986), which focuses largely on sediments in volcanic arc settings, and that of van de Kamp on the Pacific margin of North America (van de Kamp et al., 1976; van de Kamp and Leake, 1985), little information is available on the effects of provenance and tectonic setting on the composition of mudrocks.

The aim of this paper is to investigate secular changes in mudrock composition on a single crustal block, which we define as a piece of continental crust with a restricted crystallisation or accretion age, distinct from that of the crustal blocks around it. We report on the compositions of mudrocks deposited on the Colorado Province during the time interval 1.8-0.2 Ga. This time interval spans the time of formation of the crustal material, its cratonisation period, and its subsequent stable cratonic history. The specific objectives of this paper are (a) to establish whether time-related changes in chemistry can be identified in a series of mudrocks deposited

2919

2920 R. Cox, D. R. Lowe, and R. L. Cullers

q :::::: E2! ,,,,I,

::::::’

\\\ ,,,, q .\. ‘\‘\I\’ . cl

2:

t ~.,. .”

1.1-1.2 Ga

1.6-1.7 Ga

1.7-1.8 Ga

1.8-1.9 Ga

a2.5 Ga

FIG. 1. Distribution of Precambrian age provinces in North America (after Karlstrom et al., 1987). The Colorado Province has a crystallisation age of about 1.8- 1.7 Ga. The box shows the approximate limits of the study area.

on a crustal block as it evolves into a mature craton, and (b) to investigate the relative importance of sediment recycling and first-cycle input in governing mudrock chemistry on a regional scale over time.

0.6 . R4

I iiKZb*”

0.6

1.4-

1.6. . ia

. RI

1.8- l RO

2.0-

omvnk orogcny

FIG. 2. Distribution of ages of sedimentary sequences in each sedimentation interval and of major tectonic events that affected the Colorado Province, both directly (intra-block events) and indirectly (external events). Ages are taken from compilations in Kauffman ( 1977) Elston ( 1989). Hoffman ( 1989), Cox et al. ( 199 I ), and Karl- Strom and Bowring (199 1).

2. STUDY AREA: THE COLORADO PROVINCE

2.1. Crust Formation and Orogenir History

The Colorado Province (Bickford et al., 1986) is the basement complex that underlies much of the southwestern U.S. (Fig. 1). The oldest rocks in the Colorado Province are metamorphosed and strongly deformed Early Proterozoic volcanic units and immature volcanic sediments (Condie and Neuter, 1981; Condie. 1982; Bick- ford and Boardman, 1984; Boardman, 1986a,b). The mafic volcanic sequences are dated to about I .80- 1.70 Ga and are associated with 1.79- 1.74 Ga talc-alkalic intrusive rocks and felsic volcanic rocks (Silver and Barker, 1967; Anderson et al., 1971; Reed et al., 1987; Van Schmus et al., 1987). The province probably formed by the accretion of several arc-related terranes onto the southern edge of the Archean North American craton (Condie and Neuter, I98 1; Bickford and Boardman, 1984; Condie, 1986; Karlstrom et al., 1987; Silver, 1987; Karlstrom and Bowring, 1988, 1992; Van Schmus et al., 1992). The chemistry of the volcanic rocks indicates that they represent both evolved oceanic and continental margin arcs (Condie, 1986). Grogenic events accompanying accretion occurred during the approximate intervals 1.70- 1.75 and 1.6% 1.60 Ga (Karlstrom and Bowring, 1988; Bowring and Karlstrom, 1990, Karlstrom and Bow- ring, 1991) (Fig. 2). Subsequent post-cratonisation deformation of the Colorado Province was dominated by vertical movements, regional upwarping, and extensional block faulting (Grose, 1972; Mal- lory, 1972; King, 1977; Keith and Wilt, 1986; Baars et al., 1988; Sloss, 1988). with little or no metamorphism and limited involve- ment of basement.

2.2. Magmatic History

Magmatism during the time interval of interest may be assigned to three main phases: (a) magmatism accompanying accretion and early orogeny; (b) 1.4 Ga anorogenic magmatism; and (c) 1.0 Ga Grenvillian magmatism.

(a) Magmatism associated with the early history of the Colorado Province includes early, arc-related volcanism, generally consisting

Provenance of mudrocks based on their chemical compositions 2921

A Y

110

Age /Gal

GlXdteS

TOIldltCS

GlldSSCS

BUnodd volcanic sequmctsl

Manc volcantc squenences

Felslc voleante aequenas

Volcanic rocks undUTerentIated

FIG. 3. Distribution through time of rock types in the basement of the Colorado Province. The values are based on present-day amal exposure of rock types of different ages (Shride, 1967; Condie, 1981), and so represent only a crude estimate of the actual distribution of crystalline source rocks in the past, but the increasing domi- nance of granitic material is evident.

of bimodal basalt-dacite-rhyolite assemblages. These volcanic rocks and younger supracmstal sequences are intruded by tonalites and monzonites and occasional gabbros (Biclcford and Boardman, 1984; Boardman, 1986b; Condie, 1986; Reed et al., 1987; Bickford, 1988).

(b) A suite of subalkalic and peraluminous granites with ages ranging from I .46-l .36 Ga intrudes the Colorado Province basement (Anderson, 1983). These are broadly coeval with, and probably ge- netically related to, the igneous rocks of the Granite-Rhyolite Prov- inces of the mid-Continent, as described by Bickford et al. ( 1986) and Van Schmus et al. (1987).

(c) The latest magmatic event of consequence on the Colorado Province during the interval of interest is represented by the I .02 Ga Pikes Peak Batholith, which consists of granite with minor associated syenites and gabbros (Barker et al., 1975; Tweto, 1987), and by volumetrically minor I.1 Ga mafic dykes and lavas in the Apache Group and Grand Canyon Supergroup (Condie, 1981; Elston, 1989; Wntcke, 1989).

Figure 3 shows the proportions of mafic and granitic basement rock in the Colorado Province as a function of age. Early, accretion-related magmatism included a high proportion of mafic material, but later additions to the upper crust have consisted primarily of granitic plutons, with rare and volumetrically minor events of mafic volcanism (Anderson, 1989b; Bickford et al., 1989; Hoffman, 1989).

2.3. Basin-Forming Events and Sedimentation History

The tectonic setting of the Colorado Province has changed through time as it has evolved from a series of arc complexes into a mature

craton. The oldest sediments consist of immature, volcanic-arc derived detritus; the latest ones studied are the supermature deposits of the Dakota Sandstone. The intervening sequences consist of sediments at varying stages of maturity. The sequences examined in the course of this project represent the 1.7-1.8 Ga arc-related rocks, designated RO, and five subsequent sedimentation and recycling intervals, designated RI to R5 (Fig. 2).

The Colorado Province basemen? (RO) consists of arc-related volcanic rocks, generally bimodal in composition, which have been deformed, intruded by several generations of plutonic rocks, and metamorphosed to upper greenschist or lower amphibolite facies (Bick- ford, 1988; Grambling et al., 1989; Williams, 1991) . The Ash Creek Group and the Yavapai Supergroup in Arizona were sampled.

Subsequent sedimentation and recycling intervals (RI -R5) mark times during which there were both regional exposure of sedimentary rocks and widespread deposition of shallow marine sediment. The sedimentary sequences deposited during these sedimentation intervals range in age from about I .7 to 0.2 Ga (Fig. 2). The provenance and tectonic setting of each of these sequences is described in detail in Cox and Lowe ( 1995b).

RI sequences include the Vallecito Formation (Colorado), the Al- der Group and Texas Gulch Formation (Arizona), and the Vadito Group (New Mexico). These sediments were deposited in settings related to the accreting arc complexes, and on a shallow shelf which developed on top of the older volcanic arc (Soegaard and Eriksson, 1986; Gonzales, 1988; Reed, 1988; Mawer et al., 1990; Condie et al., 1992). They were derived largely by erosion of penecontempor- aneous felsic and mafic volcanogenic rocks (Gastil, 1958; Reed, 1988; Mawer et al., 1990).

R2 sequences include the Hondo Group (New Mexico), the Ma- zatzal Group and Hess Canyon Group (Arizona), and the Uncom- paghre Formation (Colorado). These sediments were probably deposited in local, fault bounded basins which formed during the Ya- vapai Grogeny (Bayne, 1987; Bowring and Karlstrom, 1990). They were derived from the volcanic arc successions and metamorphic rocks of the recently uplifted orogen and contemporaneous rhyolitic volcanic rocks (Trevena, 1979; Bayne, 1987; Harris and Eriksson, 1987; Neet and Knauth, 1989; Doe and Karlstrom, 1991).

R3 sequences are limited to the Grand Canyon Supergroup and Apache Group (Arizona). These sediments were deposited in fault- bounded intracratonic basins which formed in the latter part of the Proterozoic, approximately between 1.3-0.8 Ga (Elston, 1989). and have been interpreted to represent widespread extension and at- tempted rifting of the North American continent (Stewart and Poole, 1974; Stewart, 1978, 1982; Wrucke, 1989). Source rocks for the sediments included the contemporaneous mafic igneous rocks and the newly-exposed I .4 Ga granites, along with the sedimentary and metamorphic rocks of R2, RI, and RO (Middleton and Trujillo, 1984; Wrucke, 1989; Cox and Lowe 1995b).

R4 sequences are represented by the Tapeats Sandstone and Bright Angel Shale (Arizona) and Ignatio Quartz&e (Colorado). They record transgressive deposition across the newly-formed western margin of the North American craton ( Lochman-Balk, I97 I, 1972; Stew- art, 1972, 1982). The sediments were derived from sedimentary and metamorphic rocks representing the intervals RO-R3 and from 1.4 Ga granites.

RS sediments of the Dakota Sandstone, Mancos Shale, and correl- ative equivalents (Arizona, New Mexico, Colorado and Utah) were deposited in the Cretaceous Interior Seaway, which developed in response to uplift of the Sevier foreland fold and thrust belt (Schwans, 1988; Weiss and Roche, 1988). They are Upper Cretaceous sediments (Curtis, 1988; Molenaar, 1988; Peterson, 1988)) derived from uplifted Late Precambrian and Paleozoic marginal sequences and Mesozoic continental sequences in the thrust belt (Mackenzie and Poole, 1962; Bauer, 1989).

3. METHODS

3.1. SampIing Technique

Mudrock samples were collected throughout the Colorado Prov- ince (Fig. 4). The precise localities are given in Cox ( 1993). All


UTAH COLORADO

NEW Luaco /

RO * Rl A

R2 A

R3 0

R4 0

RS 4

FIG. 4. Location map showing study area and sample sites. Details of sample localities are given in Cox (1993).

were associated with sands deposited in shallow or marginal marine settings (Cox, 1993; Cox and Lowe, 1995b). Shallow marine dep ositional environments cover broad areas and receive sediment from many streams and by aeolian transport. The sediment is transported and mixed within the marine setting by currents and by waves, so that the final deposit represents an average of detritus from many sources. Sampling sequences from a specific depositional environment also permits comparison between sample localities and between sedimentation intervals without the need to compensate for compositional and texturai effects which might be a function of depositional setting.

3.2. Analytkal Techniques

Major element analyses were performed by Ron Hartree at the University of Ottawa. Samples were prepared as fused disks, using a mixture of 0.43 g LixCO,, 3.9 g LigBO,, and 1.3 g sample (R. Hartree, pers. commun.). The samples were analysed on a Phillips PW 1410180 X-ray fluorescence spectrometer, using matrix cor- rection coefficients given in de Jongh (1973) and Rousseau ( 1984a.b). Total Fe content is reported as Fe203,,). Average analyses for USGS standards run concurrently with the study samples are given in Table 1 a.

Trace element analyses (25, Y, Sr, U, Pb, Ga, Zn, Cu. Ni, Ti, and V) were obtained at Stanford University on a Rigaku S-max auto- mated X-ray fluorescence spectrometer with a rhodium X-ray source. Sample preparation and analysis followed methods described in Nor- rish and Chappell ( 1977). Mass absorption coefficients were calculated according to Reynolds ( 1963, 1967) and Walker ( 1973). Av-

erage analyses for concurrently run USGS standards are presented in Table lb.

The concentrations of seven rare earth elements (La, Ce, Sm. Eu, Tb, Yb, and Lu) and of the trace elements Rb, Ba, Th, Hf. SC, and Cr were determined by instrumental neutron activation at Kansas State University after methods given in Gordon et al. ( 1968) and Jacobs et al. ( 1977) (Cullers et al.. 1988). Compo- sitions of standard rocks analysed using these techniques are reported in Cullers et al. ( 1979, 1987). Standard errors are tabulated in Cox et al. (1991).

The REE chondrite normalisation factors used in this study are those of Wakita et al. ( 1971). Alternative normahsation factors (Evensen et al., 1978) have been used in other studies (e.g., Taylor and McLennan, 1985). There is little difference between the REE patterns produced: normalising to Evensen et al. ( 1978) shifts the patterns to slightly lower values of sample/chondrite.

3.3. Stat&icui MeUmds

For brevity, the statistics presented in this paper are limited to trend analysis and estimates of error on group averages; for a more extensive statistical treatment of the results see Cox ( 1993). The median is used as the summary statistic because it provides a robust estimate of central tendency for datasets drawn from a population whose distribution pattern is unknown, and for which a normal distribution cannot be assumed (Lister, 1982; Rock et al., 1987; Rock, 1988). The confidence level on the error bars (shown in Figs. 7, 8, 9, 12) varies from 94% to 98% (see Lister 1982; Appendix III). The errors were calculated using methods based on the bino- mial distribution, as described in Daniel ( 1990). Geologic discus-

Provenance of mudrocks based on their chemical compositions

Table 1. Average analyses for standard rocks used during this study. Published values are from GovindaraJu (1999).

a. Major elements, wt. %

2923

46.94 49.71 0.20 52.65 53.04 0.16 60.05 60.06 0.11 69.06 69.05 0.32

13.00 13.65 0.06 17.52 17.67 0.12 12.04 12.10 0.06 15.36 15.31 0.10

12.23 12.37 0.06 9.70 a.70 0.10 6.3( 6.26 0.06 2.66 2.74 0.02

7.23 7.36 0.04 4.*0 4.34 0.05 2.69 2.66 0.02 0.75 0.66 0.04

11.40 11.41 0.03 7.05 7.00 0.03 7.96 7.99 0.02 j.96 1.93 0.01

2.26 2.10 0.16 2.96 2.90 0.14 4.31 4.50 0.13 4.06 4.24 0.13

0.52 0.52 0.00 3.70 1.70 0.01 4.44 4.46 0.02 4.46 4.46 0.02

2.71 2.72 0.01 1.09 1.05 0.01 0.14 0.13 0.06 0.46 0.46 0.00

0.27 0.26 0.01 0.25 0.22 0.01 0.43 0.44 O.O( 0.14 0.13 0.01

0.01 0.01 0.00 0.04 0.02 0.00 0.02 0.03 0.00 0.01 0.00 0.00

0.*7 0.17 0.00 0.22 0.22 0.00 0.32 0.33 0.00 0.03 0.04 0.00

b. Trace elements, ppm

b 15.00 W.56 0.26 10.00 (6.55 0.42 (2.00 12.20 0.12

Zr 227.00 243.77 1.77 179.00 176.29 1.20 309.00 326.14 2.54

Y 20.00 19.10 0.36 27.60 25.52 0.45 II.00 9.73 0.63

6f 662.00 665.09 4.36 403.00 360.54 2.17 476.00 461.57 3.10

U 1.92 2.96 0.53 0.42 3.20 0.74 2.07 2.46 0.51

m 36.00 36.61 1.44 2.60 7.06 0.95 30.00 31.66 1.11

ci 20.00 19.96 0.47 21.00 21.09 0.73 23.00 21.76 0.33

m 66.00 60.67 1.05 105.00 106.66 1.26 66.00 66.69 0.56

al 60.00 62.53 1.73 136.00 137.06 2.02 11.00 10.46 0.56

H 16.00 14.60 0.63 121.00 117.61 1.05 5.00 5.22 0.62

TlcQ% 1.05 1.05 0.01 2.71 2.75 0.02 0.46 0.47 0.00

V 121.00 121.93 3.56 317.00 315.31 6.36 36.00 37.29 2.29

sion of this methodology may be found in Lister ( 1982). Rock et al. (1987). and Rock (1988).

In fact, aaaiysis of these data has shown that the mean and the median for each sedimentation interval are the same within error (Cox, 1993). Therefore, the data are by definition normally distrib- uted. For this reason, the analysis of trends is based on standard linear regression. The p value shown (Figs. 7.8.9, 11, 12) is the computed probability that the slope of the regression line is not zero. Values less than 0.0500 indicate that trends are significant above the .95 level; values less than 0.0100 indicate that trends are significant above the 99 level.

4. BULK ROCK CHEMISTRY AS AN INDEX OF MUDROCK COhWOSlTlON

Chemistry is the best index of compositional difference between mudrocks of different age, provenance, and diagenetic history. Because clay minerals are labile and recrystallise readily, radical mineralogical changes occur during diagenesis and low-grade metamorphism (Burst, 1959; Weaver, 1967a,b; Dunoyer de Segonzac, 1970; Weaver and Beck, 1971; Hower et al., 1976; Sudo, 1978; Eslinger and Sellars, 198 1; Weaver, 1989). These processes are largely isochem- ical: some local redistribution of material may occur, but large-scale dispersal or addition of components is uncommon

(Haskin et al., 1968; Weaver and Beck, 1971; Cullers et al., 1974; Hower et al., 1976; Ronov et al., 1977; Hower, 198 1; Riisler and Beuge, 1983; Bau, 1991) . Therefore, although mudrock mineralogy may differ from that of the deposited sediment, the bulk composition remains largely the same.

The patterns seen in the major element chemistry of the Colorado Province mudrocks reflect changes through time in the mineralogy of fine-grained detritus entering the sedimentary system; but the present-day mineralogy of the Colorado Province mudrocks is related to their diagenetic history, and can be expected to vary as a function of age and degree of recrystallisation. The rocks studied have experienced differing degrees of diagenesis, and the oldest sequences have un- dergone metamorphism (Nielsen and Scott, 1979; Soegaard and Eriksson, 1985; Harris, 1990, Gillentine et al., 1991). Consequently, chemical composition and oxide ratios are used in this discussion as indicators of original detrital mineralogy to permit direct comparisons between samples of different ages.

Oxide ratios can be used to character& differences between samples. Clay minerals and nonclay silicate minerals are character&d by very different proportions of alumina.


Table 2. Major &men1 concentrations in weight % tar mudrocks 01 Ih9 basement (RO) and 5 sedmenl recydiwg inlew&s (Rl - I%).

SWJllWlC9 Sample R LOC. t LOI so2 Al203 Fe203’ MgO CeO N620 K20 TiO2 P2O5 MnO ToMI

AC.J.IJ9.1 V.MC.523 Y.MC.524 Y.MC.525 Y.MC.526

TG.PR.156.1

TG.PR.156.3 TG.PR.157 TG.M.lBI cox2Q2

cox.203.2 cox.205 AL.TC.Sla AL.TC.519 AL.TC.520

AL.TC.522 A.V.651 A.Y.652 A.V.654 A.Y.655 A.Y.656 A.V.659 A.V.660 V.DHC278 V.DliC280 V.PF.43, V.PF.432 V.PF.434

BC4uF” SC-SW” U.LC.288 U.LC.290 U.LC.29, O.PS.127.1 WL.YJ.161.2 WL.VJ.162 WL.VJ.185 BJ.SJ.190 M.P.153 M.TB.174 MS.BC.194 cox.210 M.P.361 MS.STC.496 MS.STC.458 MS.STC.UO MS.STC.44,

DMSTCU2 DM.STCM3 MS.BC.446 DR.621

0

0

0

0

0

1

I

1

1

1

1

1

1

1

1

1

1

1

I

1

1

1

1

1

1

1

1

1

2 2 2 2 2 2 2 2 2

2 2 2 2 2 2 2 2 2 2 2 2 2 2

6 6 6 7 4

4 4 4 4 4

4

5 5 5 5

5 5 5 ,3 13 9 9 9

19 19 19 1, 17 17 17

17 (6 16 14 14

16 I5 15 15 15 15 15 14

la

1 .s. 74.2 7 12.62 365 0.86 63.78 16.60 8.68 2.19 59.79 19.36 a.56 2.50 57.94 19.35 7.95

0.75 65.33 14.65 7.65 1.54 63.76 16.60 7.95

0.40 0.09 0.14 100.75 0.71 0.23 0.08 loo.56 0.75 0.21 0.09 99.12 0.95 0.22 0.09 99.61 0.511 0.28 0.08 99.62 0.71 0.22 0.09 DO.92

2.14

2.07 3.29 2.6, 2.53

3.45 2.96 2.72 3.18

2.84 4.14 3.80 3.53 3.44 3.42 3.62 3.06 3.28 2.23 2.62 1.16 2.07 , .40

2.66

64.49

65.3, 68.07 62.69 66.14

60.76 77.57 53.85 61.56 64.82

63.75 63.50 62.12 61.72 62.56 60.96 62.57 61.00

62.73 67.50 63.06 65.33

62.73

16.06 a.24

16.29 7.58 14.61 7.61 16.43 6.15 16.33 6.70

,a.96 6.16 6.70 4.76 18.28 17.74 20.65 7.58 18.03 6.46

16.11 6.46 19.26 7.01 19.41 7.66 19.33 7.87 19.47 7.17 21.19 7.43 19.33 7.48 19.62 6.20 16.78 11.76 16.48 a.71 13.75 6.17 15.24 10.60 14.55 9.27

16.28 9.15

0.87 0.76 5.69 0.52 2.54 2.35 2.80 1.95 1.91 1.52 0.76 3.98 2.50 2.54 2.09 3.48 2.57 3.38 2.76 1.8, 2.50 2.35 2.76 1.95

1.75 0.97 3.16 2.86 1.51 0.99 3.37 2.79 1.71 0.,5 O.GU 3.26 I.64 1.04 1.49 3.03 0.66 0.25 2.64 4.12

2.14 0.28 1.15 3.43 0.30 2.14 0.02 2.33 0.07 1.05 0.92 3.56 0.09 o.la 0.1, 5.09 0.19 0.21 9.75 3.70

0.87 1.21 1 .Ol 2.73 1.13 0.27 OY 4.37 I.01 0.25 0.59 4.00 1.12 0.23 0.40 4.32 1.01 0.11 0.64 4.la

0.95 0.22 0.95 4.22 1.24 0.16 0.25 4.51 1.42 0.22 0.32 4.39 0.54 ,.37 0.99 2.69 0.90 1 .a6 0.00 4.80 1.06 1.82 1.*7 3.40 1.13 1.70 0.6, 3.50 1.13 4.09 ,.,3 1.71 1.06 0.28 0.76 3.50

0.48 0.09 0.2, 100.45 0.49 0.11 0.19 100.77 0.52 0.09 0.09 99.60 0.7a 0.15 9.12 99.73 0.63 0.12 0.02 100.14

0.67 0.16 0.21 99.37 0.34 0.1, 0.01) 99.33 2.14 0.76 0.04 101.13 0.82 0.13 0.03 99.62 0.62 0.16 0.04 99.82 o.al 0.17 023 99.49 0.75 0.21 0.05 loo.79 0.75 0.16 0.05 99.53 o.a3 0.20 0.07 99.53 0.73 0.11 0.05 99.52 0.66 0.15 0.05 loo.40 0.74 0.11 0.10 99.55 0.76 0.16 0.10 99.47 0.86 0.25 0.02 99.46 0.58 0.74 0.W 99.48 1.15 0.16 0.10 100.14 1.39 0.25 0.09 99.74 1.21 0.23 0.21 100.26

0.75 0.16 0.06 99.62

4.34 54.56 22.33 11.63 1.43 0.12 0.00 3.55 0.94 0.12 0.17 99.2, 3.45 63.26 17.96 5.3a 1.3a 0.06 0.00 4.91 9.94 0.03 0.0s 97.51 9.06 73.33 11.37 0.57 0.77 0.03 0.90 2.91 0.50 0.05 0.01 96.60 3.53 63.71 (9.23 3.96 1.52 0.07 0.00 5.42 9.00 0.94 0.03 91.53 2.10 76.27 14.46 , .36 0.35 0.03 0.00 3.60 0.66 0.01 0.01 96.87 4.6, 61.20 7.60 1.03 0.49 0.42 0.00 2.13 0.37 0.2, 0.02 98.41) 3.57 63.25 17.64 8.48 0.96 0.15 0.00 4.76 034 0.02 0.02 99.71 3.59 48.04 24.77 15.35 0.21 0.07 0.00 6.96 1.08 0.05 0.00 loo.14 4.90 50.47 17.04 19.17 2.26 1.27 0.00 2.20 0.71 0.93 0.45 1w.20 2.44 67.52 15.83 5.19 1.2a 0.6, 1.61 4.41 0.60 0.12 0.05 99.66 9.21 42.67 27.48 15.47 0.24 0.13 0.00 I .62 2.50 0.34 0.07 99.93 6.63 50.14 27.82 11.20 0.14 0.06 0.00 2.10 1.32 0.,3 0.07 99.61 3.50 69.30 14.55 7.20 1.55 0.09 0.00 2.49 0.63 0.03 0.08 99.42 2.99 66.11 77.99 4.22 1.03 0.46 I .63 3.65 0.m 0.06 0.09 90.93 3.04 67.06 15.77 5.17 0.79 0.,4 0.00 4.69 0.63 0.04 O.OJ 97.38 3.20 66.43 16.44 4.10 1.39 0.16 0.57 3.67 0.71 0.04 0.05 96.63 3.16 67.16 16.67 6.10 1.43 0.16 0.39 3.09 0.65 0.05 0.07 90.93 3.62 64.29 19.60 3.66 1.22 0.15 0.25 4.93 0.73 0.04 0.04 90.75 3.35 69.14 17.12 3.66 1.39 0.15 0.30 4.19 0.72 0.03 O.C+ 99.11 3.9, 64.3, 21.65 9.01 0.00 0.04 0.w 0.46 0.69 0.04 0.01 96.51 3.69 66.57 19.67 7.93 0.03 0.04 0.00 0.40 0.93 0.03 0.02 99.33 5.68 41.67 28.02 12.24 3.10 0.11 0.00 6.79 1.29 0.04 0.09 9923 3.78 72.24 14.79 3.50 0.68 0.10 0.W 3.54 0.40 0.04 0.05 09.12

3.59 66.11 17.84 5.36 1.03 0.12 0.00 3.65 0.72 0.04 0.05 99.21

We, therefore, define a ratio, the Index of Compositional Vari- ability (ICV) :

(Fe202 + K20 + Na20 + CaO + MgO + MnO + Ti02)/Al202,

which measures the abundance of alumina relative to the other major cations in a rock or mineral. Silica is excluded to eliminate problems of quartz dilution. Nonclay silicates contain a lower proportion of Al202 than clay minerals, relative to other constituents, therefore, they have a higher XV. In addition, there is a compositional gradient within the nonclay silicates: the ICV tends to be highest in minerals high in the weathering sequence of Goldich ( 1938), such as the pyroxenes and am- phiboles and decreases in more stable minerals such as the

alkali feldspars. The ICV decreases further in the montmorillonite group clay minerals and is lowest in the kaolinite group minerals (Fig. 5).

Because minerals show a relationship between resistance to weathering and ICV, the ICV may be applied to mudrocks as a measure of compositional maturity. Composi- tionally immature mudrocks that contain a high proportion of nonclay silicate minerals, or that are rich in clay minerals such as montmorillonite and sericite will have high values of this index. On the other hand, compositionally mature mudrocks, poor in nonclay silicates or dominated by minerals such as those of the kandite family, will have low values. Compositionally immature mudrocks tend to be found in tectonically active settings and are characteristi-


SBlpJ5llC5 sample R Loci Lol SiQ AI203 Fgo3’ MC’ CBO N@ K20 Tii Pa MnO Total

Ap.YR.470

Ap.RD.514

H.Mc.595

H.mx35

H.MC.526

S.C.545

H.TT.6ti(b

H.lT.556b

H.TT.555b

H.lTdg

H.Tr.553

H.Tr.564

H.lT.STIb

HPC.575b

H.PC.55Ob

H.PC.583

H.PC.555b

H.PC.555

LLC.254

T.J.355

T.GC.535

T.GC.539

BA.PB.377

BA.MC.531

M.Mc.532

BA.Mc.533

5APc.525

5APc.026

BAFc.527

BAPc.525

BA.Pc.535

5A.Pc.531

5A.Pc.w3

5A.Pc.534

B&PC.535

BA.Pc.537

D.Pc.217

D.Pc.224

D.TC225A

OAFa3Q

q .Fc250

D.BY314

DEB.%37

o.cs.421

3

3

3

3

3

3

3

3

3

3

3

3

3

3

3

3

3

3

4

4

.

4

4

4

4

4

4

4

4

.

4

4

4

4

.

4

5

5

5

5

5

5

5

5

25

27

22

22

22

22

22A

22A

2%

2u

2a

225

225

23

23

23

23

23

4a

25

31

31

37

35

35

35

37

37

37

37

37

37

37

37

37

37

42

42

43

44

42

45

45

45

4.12

5.55

1.55

5.39

5.39

2.75

5.47

7.32

5.05

2.14

3.37

1.72

2.09

7.74

12.45

4.50

5.50

2.95

4.55

3.72

5.52

3.53

5.52

5.75

5.05

5.45

5.44

5.12

5.55

5.54

5.14

5.40

5.04

5.20

5.01

5.34

7.53

5.53

5.14

5.40

,523

5321

55.57

63.34 53.34 55.w

55.57

55.35

5524

55.54

55.25

55.55

W.52

55.50

WJn

55.14

55.40

55B2

55.44

52.55

45.55

53.57

54.42

51.57

50.83

5353

55.41

55.31

55.34

54.51

53.75 54.55 52.47

52.77

54.94

54.55

45.15

5..,5

70.72

5S.50

54.51

74.15

5l.W

70.3s

51.05

m23

57.55

25.45 12.98 0.71 0.07 0.35 7.95 1.14

15.35 1.45 0.40 0.08 0.00 1O.W 0.05

13.59 5.59 0.79 0.25 0.00 5.49 0.53

(1.04 3.55 2.47 2.05 0.00 4.55 0.51

11.04 3.55 2.47 2.99 0.W 4.55 0.51

15.79 5.54 0.53 0.16 0.W 5.45 0.55

12.79 4.73 4.75 4.25 0.W 7.w 0.50

12.57 4.97 4.14 3.37 0.W 5.02 0.55

12.59 5.25 4% 2.39 0.00 7a7 0.72

11.55 5.54 1.04 0.45 0.00 5.54 0.95

17.94 12.55 1.55 0.53 0.00 524 0.57

14.40 5.52 0.55 024 0.00 5.w 0.55

17.44 7.71 0.99 023 0.00 0.51 0.75

13.03 5.95 4.00 3.57 0.00 5A5 0.71

11.09 5.55 5.01 7.15 0.00 5.73 0.w

20.71 5.51 1.54 0.47 0.00 7.75 0.52

13.35 5.12 3.97 4.1. 0.05 5.55 0.75

13.53 7.44 1.37 0.47 0.w 5.70 0.73

13.51 5.73 1.70 0.55 0.00 7.70 0.70

19.07 3.25 1.48

24.71 10.11 1.01

21.42 5.95 0.97

21.50 0.35 1.15

24.07 4.43 1.45

27.25 3.79 1.15

21.77 5.5Q 2.07

22.40 5.15 0.75

23.55 4.24 1.12

22.29 5.59 2.02

21.25 7.71 2.31

22.29 7.35 2.35

22.39 7.71 2.05

2l.W 9.33 2.50

22.41 7.92 2.37

20.39 5.79 2.72

22.40 5.02 2.39

31.33 2.56 0.57

22.3. 5.30 1.75

0.15 0.00 5.02 1.04

0.15 0.w 7.15 1.W

0.15 0.w 0.14 0.55

0.07 0.W 5.47 1.15

0.52 0.w 5.19 1.04

0.15 0.w 5.U 1.01

0.w 0.w 0.15 0.93

0.55 0.00 5.99 1.05

0.25 0.00 5.96 0.54

0.20 0.00 5.55 1.04

0.15 0.w 5.79 0.55

0.22 0.w 7.05 0.55

0.15 0.00 5.3Q 0.55

0.15 0.00 723 0.92

023 0.00 7.25 0.94

0.70 0.w 7.22 1.03

0.17 0.W 5.43 0.96

0.15 0.w 5.79 1.58

l1.55 0.35 0.07

17.05 2.14 1.14

13.21 5.04 0.13

5.35 5.03 0.02

19.15 2.W 1.11

13.35 1.51 1.15

21.02 2.05 0.53

10.04 3.12 0.53

13.29 2.37 0.55

0.15 0.00 7.10 0.50

0.17 0.W 0.35 0.W

0.31 0.00 2.5s 0.55

0.03 0.00 1.00 O.*o

0.77 0.00 123 0.35 0.13 0.00 2.55 0.72

0.07 0.00 2.42 0.w

0.15 0.00 2.42 0.74

0.51 0.w 1.95 0.W

0.15 0.00 2.15 0.55

0.05

0.12

0.13

0.15

0.15

0.50

0.13

0.13

0.15

0.25

0.55

0.15

0.15

0.15

0.12

0.22

0.20

0.22

0.15

0.03

0.25

0.05

0.10

0.12

0.15

0.08

0.42

0.12

0.07

0.09

0.10

0.05

0.12

0.07

0.07

0.03

0.12

0.10

0.01

0.m

0.05

021

0.12

0.01

0.03

0.05

0.04

0.01 0.0,

0.02

0.12

0.13

O*Iz

0.10

0.09

0.00

0.02

O&?

0.01

0.01

0.10

0.15

0.02

0.11

0.03

0.03

0.02

0.W

0.00

0.02

0.02

0.02

OXa

0.05

0.02

0.04

0.03

0.03

0.01

Ox)4

01)3

0.05

ON

0.02

0.03

0.01

0.0,

0.02

0.11

Ox)?

0.01

on1

0.05

0.01

55.45 55.02 94.53

99.97

94.53

OS.07

99.45

99.S2

98.15

88.92

101,02

9950

00.40

em4

ml.55

loo.35

On.45

00.35

55.53

90.34

99.W

99.33

SOB5

99.15

95.92

SO27

9925

Se.53

90.72

99.57

10020

loo.71

lW.33

10023

Q&s3

99.43

00.01

55.55

PO.13

98.m

oo.27

55m

89.04

57.07

a%2O

a5.73

95.00

tally first-cycle deposits (van de Kamp and Leake, 1985). Compositionally mature mudrocks characterise tectonically quiescent or cratonic environments (Weaver, 1989) where sediment recycling is active, but may also be produced by intense chemical weathering of first-cycle material (Barshad, 1966).

Another index of mudrock composition is the ratio KrO/ A1203_ Clay minerals and feldspars have markedly different values for this ratio (Fig. 6)) and so it can also be used as an indicator of the original composition of ancient mudrocks.

Trace element chemistry of mudrocks has been used in several studies to compare mudrocks of different ages or provenance or to establish the relative contribution of fractionated igneous material to the sedimentary system (e.g., Ronov and Migdisov, 1971; Taylor and McLennan, 1981, 1985; Mc- Lennan and Hemming, 1992). Many trace elements are ex-

tremely insoluble in aqueous systems, and tend, therefore, to be transferred from source rock to sediment without significant fractionation (Balashov et al., 1964; Nesbitt, 1979; Da- vies, 1980). Trace elements do not form mineral frameworks, but generally occur in association with clay minerals as sorbed particles on surfaces or included in interlayer cation sites; therefore, their occurrence in mudrocks is not strongly related to mineralogy and bulk composition. Further, most hydrothermal fluids have very low concentrations of insoluble trace elements such as the REE; hence, alteration of whole-rock trace element patterns during hydrothermal alteration and metamorphism is generally ineffective, except in cases where the ratio of water to rock is much greater than lo*- lo3 (Bau, 199 1) . Water-rock ratios on a regional scale during regional metamorphism are typically close to 10” (Ferry, 1983, 1988; Wood and Walther, 1986). For these reasons, ratios of low-


.Ol .l 1 10 100

+ Pyroxcnc +

+ +

+ + +

+ +

+ + + +

Am$hibde + + I

I Amphibde

I

4

,

+

+

Mite l Muscwite +

+ + + + + + + + +

K-spar

+ MuSCovite

l MOntmorlllonitc + Mite

+ r.4ontmwillonite +

Ksdinite +

1000

.Ol .! 1 10 100 1000

Index of Composfttonal Vartabtlity

FIG. 5. Values for the Index of Compositional Variability (ICV) [(Fe,O, + KaO + NaaO + CaO + MgO + MnO + Ti@)/Al,O,] for selected rock-forming minerals. Crosses represent individual data values. Where a mineral name appears beside a data point, the value refers to that mineral. The arrows show the range of values for certain mineral groups; specific values from these groups are given by the crosses that are unaccompanied by a mineral name. Data from Deer et al. (1966). See text for explanation.

solubility trace elements in mudrocks generally reflect those of the source rocks (Cullers et al., 1975; Taylor and Mc- Lennan, 1985), making such trace elements a valuable tool for provenance analysis.

5. RESULTS AND DISCUSSION

5.1. Major Elements

The major element data are shown in Table 2 and Fig. 7. Statistically significant characteristics of the data fall into three categories: ( 1) progressive decrease or increase in abundance with time; (2) excursions from progressive trends; and (3) dog-leg patterns superimposed on an overall increase or decrease. In the following sections, these data will be presented, the mineralogical and provenance controls on the

trends discussed, and the trends analysed in terms of the tectonic evolution of the Colorado Province.

5.1.1. Progressive secular trends

A first-order characteristic of the Colorado Province mudrocks is that several of the major oxides-FezOs, CaO, NazO, and MnO-decline significantly in relative abundance through time (Fig. 7). The average ICV (Index of Compositional Variation) for Colorado Province mudrocks also declines (nonlinearly) as a function of time (Fig. 8). In contrast, KrO shows a marked increase in relative abundance.

A&O3 shows a statistically significant but poorly-defined increase in average abundance. An interesting feature of the A&O3 data distribution, however, is that in several respects it is the reverse of that of SiOr. Although SiOr shows no progressive trend, there are large, statistically significant fluctuations in its abundance through time. Samples from RO, Rl, and R2 are statistically indistinguishable, but R3 and R4 samples are low in silica, and RS mudrocks are on average richer in silica than any of the older samples (Fig. 7a). Fluctuations occur in the A1203 data for R4 and R5 which are the reverse of those seen in the SiOz data.

1.0 0.2 0.4 0.6 0.8 1.0

l

!t2%%2- Range of kkhpr VahIea

oithod~se +

Addaria +

Mcrocllne +

Whodare +

Microdine +

Microcline +

Smidim +

Mhoclsse +

+ Otthake

+ Microcline

+ !alibne

+ Anathoclase

+ lllite

+ tlydrcmwscovite

+ Halloysite

+ sqmite

l Beidallite

t Montmuillonite

t Kodinite

+ Vermiculite

0.0 0.2 0.4 0.6 0.8

K2O/N203

.O

FIG. 6. Values of the K20/A1203 ratio for K-feldspars and clay minerals. Crosses represent values for the specific minerals indicated. Data from Deer et al. ( 1966).


Because the dataset is closed (components sum to lOO%), the close negative correlation between the alumina and silica data implies that large fluctuations in silica content may be masking more subtle trends in A&O3 abundance. To eliminate the effects of silica and to evaluate whether there are hidden

trends in alumina content in these samples, AlrOt ( = alumina as a percentage of the total analysis minus SiOz) was calculated. In contrast to the raw alumina data, there is a marked increase in the average abundance of A&O: through time (Fig. 9).

X1.2. Excursionsfrom progressive trends

R3 mudrocks are commonly enriched in MgO, CaO, and KrO relative to those from other sedimentation intervals (Fig. 7d,e,g). They have high values for the ICV and KrO/Alr03 (Fig. 8). They are low in SiOz (Fig. 7a), and in A&O: (Fig. 9).

Values for K20/A1203 (Fig. 8b) exceed those for illite, and lie in the range of values for alkali feldspars (Fig. 6). Values for the ICV (Fig. 8a) include the upper end of the range for feldspars (Fig. 5), but most samples have values well in ex- cess of 1 (Fig. 8a). These data indicate that ( 1) the potassium content of these samples is controlled mainly by feldspar, and (2) in addition to feldspar, the original muds contained abundant mafic high-ICV minerals, either as detrital grains or in lithic fragments. Both detrital feldspar and volcanic lithic fragments are observed in thin sections of R3 mudrocks ( Cox, 1993 ) . The low values for AlrO: (Fig. 9) reflect the concomitant decrease in the proportion of clay minerals in R3 mudrocks.

R5 mudrocks have anomalously high SiO* contents relative to the other sedimentation intervals (Fig. 7a), whereas the relative abundance of A120, is low (Fig. 7b). However, as has been discussed previously, the low AIrOr values are an artifact of the increase in silica; and AlrOT data demonstrate that, when the effect of silica is corrected for, RS samples are not anomalous with respect to alumina content (Fig. 9). The high silica contents of R5 mudrocks are, therefore, attributed to quartz dilution. This is consistent with data from the lat- erally equivalent Mowry Shale in Wyoming, in which ele- vated silica contents have been shown to be due in part to the presence of abundant detrital quartz (Davis, 1970). Silt-sized detrital quartz is commonly observed in thin sections of R5 mudrocks (Cox, 1993).

The low KrO content of these samples (Fig. 7g) is ascribed to intense chemical weathering. During the Cretaceous in the southwestern United States the climate was subtropical and humid, and weathering was intense (Bejnar and Lessard, 1976; Kauffman, 1977). These conditions, which promote leaching of soils and removal of alkalis in solution, provide the best explanation for the radical decrease in KrO abundance and low overall concentration of oxides other than SiOz and A1203 in R5 mudrocks. Kaolinite is very common in Cre- taceous mudrocks, and its abundance has been interpreted to reflect efficient chemical weathering (Finkelstein et al., 1990; Leckie et al., 1991).

5.1.3. Dog-leg overprints on progressive trends

For some oxides, statistically significant progressive trends may be resolved into two distinct subtrends. MgO, CaO, and ICV values decline, are rejuvenated in R3, and then decrease

again (Figs. 5, 6a). AlzO: values increase from RO to R2, decrease in R3, then rise again from R3-R5 (Fig. 9). These distributions are interpreted to mean that there is episodic input of first-cycle material with high ICV values, followed by periods dominated by recycling and reworking of this material to produce mudrocks with progressively more mature compositions. The basis for this interpretation will be discussed below.

The KrO/AlrO, ratio reaches a maximum in R3 and declines thereafter, and is interpreted to reflect an initial phase in which little K-feldspar was available, followed by massive input of granitic material in R3, with subsequent recycling- dominated sedimentation.

5.2. Interpretation of Major Element Trends in Colorado Province Mudrucks

The overall picture that emerges from the data is that older Colorado Province mudrocks tend to have highly variable compositions and younger ones to have more restricted compositions, dominated by potash and alumina (Fig. 7). Super- imposed on this are compositional anomalies, or second-order excursions from the general trend, as discussed above.

There is a progressive decrease in average LO1 values in older samples (Table 2), which is the expected consequence of loss of volatiles through compaction and dewatering. This is the only compositional trend in these rocks which may be attributed to diagenesis: in all other cases where a compositional trend is present, it is the opposite of that which would be predicted to result from diagenetic alteration. Generally, the oldest samples have the highest concentrations of the most mobile elements. The trends visible in the data must, therefore, be the result of real differences in original composition between mudrocks of different ages.

5.2.1. Causes of major element variation

The major element composition of mudrocks reflects their primary mineralogy, which is sensitive to the intensity of chemical weathering. Progressive weathering during erosion and transport tends to increase the abundance of cation-poor clays and oxides of iron and aluminium at the expense of both nonclay detrital minerals and compositionally complex clays (Barshad, 1966; Tardy et al., 1973; Heckroodt and Biihmamt, 1987; Middleburg et al., 1988; Weaver, 1989). The smectites, chlorites, and other mixed-layer clays which house Ca, Na, Fe, and Mn degrade quickly during weathering, breaking down into compositionally more simple clays such as the kan- dites. Elements released during this process tend to be leached, and the ultimate product is a mud depleted in soluble components and enriched in Al and Fe (Malcolm et al., 1969; Hayes, 1970; McKeague and Brydon, 1970; Herbillon and Makumbi, 1975; Potter et al., 1975; Fanning and Keramidas, 1977; Weaver, 1989).


J lsio2] a, pco.5915 - 5.

1

p = 0.0602

I . . I *.

(J * ’ p= 0.2655

15.

15 _

I..

1s.

10.

(1.

4.

, p = ,

4.

5.

1.

I

. *

1 1 : . 0,

11D Rl W R3 RI l?5

p = 0.0001

R) Rl FQ. RS RI RS

FIG. 7. Abundance of major elements as a function of sedimentation interval. Data are recalculated volatile-free Dots represent individual data Points. Circles represent group medians. Vertical bars represent errors on the estimated averages at a confidence level of -95% (see text). The p statistic shown on each graph measures the probability that the slope of a regression line through the data is zero. The smaller the number, the higher the probability that the line slope is non-zero: i.e., data distributions with p values of 0.0500 or less show time-related trends that are significant at or above the 95% level. Data distributions with values greater than 0.0509 do not show significant trends. The approximate range of compositions for continental island arc mudrocks, based on data from Bhatia (1985) and van de Kamp and Leake (1985) is shown by the thick black line on the left side of each graph.

Although K has a high aqueous solubility, it tends to be and Pickering, 1983). Therefore, muds derived by recycling

conserved in mudrocks because of the chemical stability of of older sediments generally contain a high proportion of illite illite. Illites are resistant to weathering and are quite stable in (Potter et al., 1975; Potter et al., 1980), and are consequently soils except under extreme weathering conditions (Not&h very potassic.

Provenance of mudrocks based on their chemical compositions

(9)

EEI . p = O.oool

RO Rl IU. R3 R4 RS

2929

FIG. 7. (Continued)

Tbe abundance of MgO, MnO, CaO, NarO, and FerO,( t) in rocks of the basement (RO) and early cover (RI ) is consistent with the immature character of these sediments, which were largely derived from arc-related volcanic and plutonic rocks (Anderson, 1989a; Batter and Williams, 1989). The average ICV values for RO ( 1.1) and Rl ( 1 .O) (Fig. 8a) exceed the range of values for the common clay minerals (0.03- 0.78) and feldspars (0.54-0.87) (Fig. 5), indicating that these samples contained abundant non-clay silicate minerals with high ICV values.

The subsequent decline in the relative abundance of CaO, Na,O, MgO, and Fe20, probably reflects the decrease in both nonclay silicate minerals and compositionally immature clay minerals (such as the montmorillonite group minerals) in Col- orado Province mudrocks. Mudrocks from subsequent sedimentation intervals (with the exception again of R3) have on average lower values (Fig. 8), approaching or within the range of clay mineral ICV values, indicating that later mudrocks contained lower proportions of nonclay silicates, and were dominated by clay minerals. The increase through time

in the relative proportion of KzO and A&Or is likewise interpreted to reflect an increase in the proportion of compositionally mature, alumina-rich clay minerals.

The overall trend away from compositions rich in Ca, Na, Fe, and Mn, towards more compositionally mature, potassic mudrocks is, therefore, consistent with the emergence of a recycling-dominated sedimentary system during the tectonic evolution of the Colorado Province.

These patterns are unlikely to simply reflect erosion of progressively more felsic basement. In the first place, the values for K20/A1203 for several intervals, most notably R4 and R5, are well below those for feldspar, indicating that little or no detrital K-feldspar was present in these sediments. Secondly, TiOZ abundance is much lower in felsic than in mafic rocks, so that mudrocks derived from progressively more felsic source rocks would be expected to have decreasing amounts of TiQ. In contrast, TiOl in Colorado Province mudrocks remains approximately constant through time (Table 2). Ti- tanium is a very insoluble element, which tends to be conserved in mudrocks, and its distribution in these samples im-

plies that recycling of older Ti-rich detritus is an active process in the Colorado Province.

The pattern of increasing overall maturity is complicated by excursions from the compositional trends, mainly at R3, and by the dog-leg patterns for some oxides. These may be explained by episodic rejuvenation of the sedimentary system by basement uplift, which provides an influx of first-cycle material. This interpretation is supported by a consideration of the differences between the composition of the R3 and RO/ Rl mudrocks.

Both the oldest Colorado Province mudrocks (RO and Rl )

and those from R3 are characterised by a significant proportion of (presumably) first-cycle silicate detritus, indicated by the high ICV values and the abundance of Ca, Fe, Mg, etc. Examination of the ICV and the K20/A120~ ratio, however, indicate that there are distinct differences in the composition of the nonclay silicate fraction in the RO/Rl and R3 mudrocks; the older mudrocks have a high average ICV, but low values for KzO/AlzOs, in contrast to the high ICV and high K20/A1203 of the R3 samples. This indicates that the nonclay silicate material in the R3 rocks was dominated by K-feldspar, where as the RO and Rl samples contained little K-feldspar but an abundance of sodic, calcic, and ferric silicate minerals.

Mudrocks of RO and Rl were derived from erogenic arc- related source rocks, including a high proportion of mafic volcanic rocks, at an active plate margin (Gastil, 1958; Reed, 1988; Mawer et al., 1990). R3 rocks, on the other hand, were formed on stable continental crust, and received detritus from mid-Proterozoic post erogenic granites, which were exposed throughout the Colorado Province by Late Precambrian uplift (Elston, 1989; Wrucke, 1989). In the RO and Rl rocks the primary constituents have not survived metamorphism and are represented now by biotite, chlorite, and iron oxide; but in R3 preservation is better and K-feldspar can be detected both by XRD analysis and in thin section (Cox, 1993).

The difference between the first-cycle component in RO/ Rl and R3 is nicely demonstrated by Fig. 10, which shows the averages for each sedimentation interval on a molar proportions plot of A1203-CaO + Na20-K20 (Nesbitt and Young, 1984). The data points define a dog-legged track from a start-


5 Li i

(a)

=J”‘““’

0.0

Ro RI m R3 RI R5

as (W

Ip=o.o0571

aoJ 110 Rl R2 R3 R4 R5

FIG. 8. Major element ratios (volatile free) as a function of sedimentation interval. Small dots are individual data points. Circles represent group medians, and error bars represent ~95% confidence intervals about the averages. For explanation of p statistic, see caption to Fig. 7. The thick black line on the left side of each graph shows the approximate range of compositions for island arc mudrocks (data from Bhatia, 1985, and van de Kamp and Leake, 1985). (a) The Index of Compositional Variability (ICV): (Fe+r03 + KrO + NarO + CaO + MgO + MnO + TiO#Al,Or. (b) Ratio of KrO to AlrOs.

ing point close to the composition of tonalite, towards the muscovite-kaolinite join, with an excursion at R3 to compositions close to granite. In effect this track is tracing the tectonic development of the Colorado Province sedimentaty system from an arc-dominated period, through a period of reworking to a major basement uplift event, followed by a long interval of recycling-dominated sedimentation.

5.3. Trace Elements

Complete trace element analyses are presented in Table 3. Younger Colorado Province mudrocks tend to be enriched in the incompatible high field-strength elements (HFSE) and depleted in compatible transition elements relative to older samples. This tendency is illustrated by the ratios in Fig. 11. There is no correlation between the aqueous solubilities of the ele-

10

I 80

F, 80

4 a 40

20

0 I-

/p=O.O699 1

FIG. 9. Abundance of Al,O: (see text for derivation), plotted at function of sedimentation interval. Dots are individual data poin Circles represent group medians, and error bars represent 95% co fidence intervals on the averages. For explanation of p statistic, s caption to Fig. 7. The thick black line on the left of the graph shol the approximate range of compositions for island arc mudrocks (dc from Bhatia (1985) and van de Kamp and Leake (1985)).

ments concerned (Riley and Skirrow, 1975; Broecker a~ Peng, 1982) and the trends in the data. Although some hig solubility elements (e.g., Sr, Ba) are depleted, other elemer that are mobile in weathering systems (most noticeably I increase in abundance through time (Table 3). It is, therefot difficult to invoke progressive weathering or any recyclin

CaO + Na20 K2C

FIG. 10. Average for each sedimentation interval shown on a ma proportions plot of Al,Or-CaO* + Na@-KzO (Nesbitt and You 1984). CaO* is defined as CaO in silicates only. However, there v in this case no objective way to distinguish carbonate CaO from I icate CaO, so total CaO is plotted here. This is justified on the ba that none of the samples appeared calcareous, and all samples I one contained less than 5 wt% CaO. The effect of inclusion of nc silicate carbonate would be enhance the apparent immaturity o sample by pushing the data point towards the CaO + NarO corr Values for average tonalite and average granite are from Nesbitt I Young (I 984).


lo-

8-

6-

(a)

FLa/sc

p = 0.0001

I

n .

4- * : i . I

: : * : - ! .

2. i i i i

O Rb Iti : m lu R4 R.5

0.8

0.6

J I

F&J IIt m R3 II4 R5

4-

3-

2-

l-

o-

(b)

p/SC 1 p = 0.0001

: : I

I i

i I i

!

i ! : . . .

m RI SU R3 R4 RS

O*Or;D FtG. 11. Selected trace element ratios for Colorado Province mudrocks. For explanation of p statistic, see caption to

Fig. 7. Complete analyses a~ presented in Table 3.

related process. Another possible control, increases in the amount of organic material in the sedimentary system through time, is also unlikely, because those elements analysed which have an affinity for organic material have dissimilar distributions: U increases through time, V decreases, and Ni shows no significant trend (Table 3).

The distribution of the trace elements seems on the whole to be related to their igneous chemistry rather than their aqueous or organic chemistry, which implies that the data reflect

the input of first-cycle material, and that the increasing abundance of HFSE relative to transition metals records an increase in granitic source rocks through time.

Several of the incompatible elements (Rb, Th, Hf, Zr, U) show significant progressive increase in abundance over time. In contrast, the compatible elements tend to have more irreg-

ular distributions, and do not show progressive increases or decreases through time (Table 3). Incompatible elements increase because they are being actively added to the sediment mass through time as the first-cycle input becomes dominated by granitic material (Fig. 3). Input of compatible trace elements to the sedimentary system, however, becomes negli-

gible over time; but because of the low solubilities of the metals involved, little is fractionated into the dissolved load of the transporting medium as older mudrocks are recycled. Low solubility transition metals such as Cr and SC, which were introduced to the sedimentary system mainly in RO or Rl time, will tend to be retained by clay particles during recycling (Ermolenko, 1972; Roaldset, 1973, 1978; Davies, 1980; Fleet, 1984; Salomons and Forstner, 1984; Horowitz and Ehick, 1987; Middleburg et al., 1988; Horowitz, 1991; Keizer and Bruggenwert, 1991). Therefore, in spite of the decline in input of these metals, their time-related decrease in concentration is not significant.

5.4. Rare Earth Elements

Rare earth element analyses are presented in Table 4. Eul Eu *, which measures the size of the europium anomaly (see caption to Fig. 12 for derivation), shows a marked decrease over time, and ZREE,, ,’ which is the sum of REE in a sample

’ Subscript n denotes data normalised to chondrites.


sequence Sample R Lot.’ Rb Ba Th HI SC Cd iY Y S U Pb Zn Cu N V

Ac.J.129.1

Y.Mc.S22

“MC524

“MC.525

Y.MC.526

TG.Fl3.156.1 TG.PR.156.3 To.m.157 ni.m.e.7

lG.M.lM

COX.202

COXZ02.2

Co)1205

“DnC.2,8

“DHC5M

“.PF.4%

Y.PF.422

V.PF.424

NTC518

urc51e

UTC520

ALTCJP

A_“.251

cLY.E.2

cLY.c.4

A”.155

A.Y.Ed

~cy.652

LY.660

EC-4UF”

KSSUF”

O.P5.,2,.,

0.PS.1272

Y.P.K.2

u.lB.,w

W..YJ.,212

WL.YJ.122

W,..YJ.,8S

BJ.&l.,w

M2.EC.123

usEC.,24

COXZW

“.LC228

u.LC.220

U.LC201

U.P.22,

YSslcus

YS.slC~

us.srC.442

YbsrCY,

CM.STCyt

CMbTC.443

MSDCAM

D.R.wl

0

0

0

0

0

,

1

1

1

1

1

1

1

I

1

1

,

,

1

1

,

,

1

,

1

,

,

,

,

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

1.3 7.0 5.0

291) 5.5 2.6

802 7.2 2.2

12M (1.2 42

594 6.2 2.2

224 7.2 2.6

?a 4.0 1.1

07s 1.1 2.,

I77 3.7 2.0

413 0.1 3.2

64, 7.5 2.5

940 ,4.3 2.9

71, 11.7 4.0

226 6J 2.5

546 52 a.5 ,sce 4.6 2.1

22s ,,., e.0

,590 72 2.2

10

17

2,

22

,.

17

23

22

17

2

21

12

12

2

24

15

18

2,

20

42

,s

1,

n

1,

16

II

,7

,a

18

10

12

20

IS

1,

42

16

12

21

20

,2

1,

,,

1.

,2

24

12

12

12

15

1.

,?I2

,,

22

12

7

M

75

142

66

II

30

2,

50

2a

m 04

76

35

86

59

24

18

,2

,2

,22

99

rn

28

82

u

22

77

90

91

I,

Lo

58

45

162

e4

12

(50

84

45

26

49

e

62

126

40

22

56

7,

22

(24

20

128

27

223

154

142

190

loo

,I.

92

es 123

,e 150

,a0

,?a 101

152

(1.

316

322

317

222

2,o

15,

(6s

,a2

202

22a

190

108

196

205

,,*

172

22,

(22

152

,ow

.%a

2s

290

15,

222

222

2m

220

210

270

4,2

2,.

267

255

2M

2yT

2r.a

2%

57l

208

.92

(7

,o 20

,2

IS

12

12

17

,o 1,

22

25

,2

24

22

62

5,

M

70

20

22

21

28

22

2,

2s

24

22

26

2.

45

25

10

7

172

72

22

4,

62

511

22

28

M

5,

4s

2s

22

25

a

45

.,

56

24

w

2s

relative to the sum in chondrites, increases in younger sam- fractionated igneous material in the source rocks for Colorado ples. Province mudrocks.

The rare earth element patterns support the hypothesis that trace element compositions track changes in the composition of crystalline input to the sedimentary system (Fig. 12). Eu- ropium is not fractionated during weathering or diagenesis relative to the other REE (see McLennan, 1989, for review); therefore, the increasing size of the europium anomaly in younger mudrocks must reflect input from source rocks with increasingly large Eu anomalies or from an evolving mixture of rocks with differing Eu anomalies. The REE are strongly incompatible elements which are concentrated in partial melts, and so should be enriched in sediments derived from granitic rocks. The increases in total REE content relative to chondrites, therefore, reflect the increasing importance of

5.5. Diierences between Major and Trace Element Behaviour in Mudrocks

The conclusion that trace element distributions are controlled by the composition of first-cycle input is at odds with the major element data, which are sensitive to the relative proportion of first-cycle and recycled material in addition to discriminating between mafic and felsic crystalline input. This decoupling of the behaviour of the major elements and the insoluble trace elements may be explained by their differing susceptibilities to fractionation during weathering and trans-

port. Previous studies have shown that the major element

,I I‘ 270 .,

115 4.2 6

272 2.0 1,

MO 2.0 II)

588 2.2 17

546 2.. 18

222 2.2 17

1M 2.0 10

163 3.1 1,

38 2.2 21

7, 1.2 22

909 ,.D 22

2, 1.7 15

172 2.2 22

72 2.9 ,o

247 21 14

22s I.0 22

2, 2.8 12

105 2.1 12

1Y 2.7 ,*

197 2.2 27

(51 4.2 2.

I64 2.2 22

151) 2,s 18

127 2.5 ,2

07 .A 20

112 4.3 18

09

15

,m 115

2.8

00

62

58

215

27

lo2

22

1w

52

24

22

127

14,

55

5a

52

,a3

B,

64

72

22

72

87

121

2.

(22

(22

10

,2

i-J

22

62

12

172

24

62

121

s2

2

2,

te

04

22

74

67

6,

,2

2s

,a2

ee

2‘

19

23

42

64

Y

*2

57

24

28

27

4

5

22

10

s

,2

I,

62

b(

,,

44

,6

5

te

e

,,

(2

I

1,

17

I2

,4

IO4

44

I,

22

8

e

142

e

1,

12

22

12

24

2

H

22

0

m

3

13

,a

28

14

,.

1.

m 120

143

107

120

12.

122

142

22

122

51

20

m

222

200

100

12,

262

112

‘06

26

11,

110

,w

to2

w

104

(02

102

,s,

107

(22

(04

124

20

1,.

12,

22

61

6,

57

0

27.

10s

u

4e

m

22

22

A

e

4

I22

I

.I


sequence Sample R Lot: Rb Ba Th HI SC Cr D Y Sr U Pb Zn Cu N ” -__ _

Apwb G,Md canyon 6.

Glnd Canyon 6.

Gmld canyon 6.

Gmdearqon 6.

Gnndemp 6.

Glnd Cmpn 6.

Grnd eanpn 6.

Glnd eanyvn 6.

Gmn! eanyen 6

GrmldCurpn s Gmld cmpn s. Gmmldcmp”S

Grad chlycm s. Grmd Cmyun 6.

Glmd callynn 6.

YEDlLW

I(lnacicl

T-

composition and mineralogy of fine-grained sediments are

governed more strongly by the extent of weathering than by source rock composition (Barshad, 1966; Tardy et al., 1973; Heckroodt and Btihmann, 1987; Middleburg et al., 1988; Weaver, 1989). In contrast, soils and mudrocks tend to pre- serve the signatures of the parent rock for low-solubility trace elements, even under intense weathering conditions (Bal- ashov et al., 1964; Cullers et al., 1975; Aubert and Pinta, 1977; Nesbitt, 1979; Davies, 1980; Cullers et al., 1987; Marsh, 1991).

As discussed previously, interstitial cations are removed from clay minerals as weathering prooeeds. The rate of release varies as a function of the chemical stability of the host mineral, and the rate of removal in solution varies as a function of the aqueous solubility of the ions involved; but in general, weathered clays evolve towards lower ICV values. Because

of the interstitial cation loss experienced by weathered clays, a very large volume of first-cycle detritus is required to offset the effects of weathering and transport on recycled material. Qualitative calculations show that mudrocks can absorb a significant volume (up to about 25% ) of first-cycle material and still show major element trends that reflect net cation loss due to weathering. The low-solubility trace elements on the other hand are not prone to removal in solution, and so their concentrations in a sediment are unlikely to change much as a consequence of weathering. Consequently, their abundance may be noticeably affected by volumetrically minor first-cycle input.

These results confirm the usefulness of trace elements in studies that are concerned with basement composition. They also indicate that trace element compositions of mudrocks are quite sensitive to variations in local or regional basement


Table 4. Rare earth element concmtrations in ppm for Cotorado Province mudrocks

S~UWl% SWYlple I7 LOC.' La ce Sm Eu Tb Yb LU ZREE

AC.J.139.1

Y.MC.523

Y.MC.624

Y.MC.525 Y.MC.526

TG.PR.158.1

TG.PR.l%.3

TG.PR.157

TG.PFLW

TOM.164

coxza?

cox.2cG.2

coxzO5

V.oHc276

V.c+lc290

V.PF.431

V.PF.432

V.PF.434

ALTC.616

ALTC.519

ALTC.520

ALTC.622

A.Y.%l

A.Y.652

A.Y.653

A.Y.665

A.Y.666

A.Y.669

A.Y.660

SC-UIF~

SC-6u!=-

O.PS.127.1

0.PS.127.2

kw.163

M.TS.174

WL.YJ.161.2

W-YJ.182

bL.YJ.165

SJ.SJ.180

uS.SC.193

WS.SC.194

Cox.210

U.LCZ%

U.LC200

U.LC291

M.P.%l

MsSTC.436

MS.STC.436

MS.SX.440

uS.srC.141

DYSTC.442

DMsTC.443

MS.DC.446

O.R.%2(

0

0

0

0

0

I

1

1

1

1

1

1

1

1

1

1

1

1

1

1

1

1

1

I

1

1

1

1

1

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

6

6

6

6

7

4

4

4

13

13

9

9

9

4

4

4

4

5

5

5

6

6

6

5

11

11

16

16

17

17

17

17

(4

$4

14

19

19

19

16

15

16

(6

15

16

15

14

16

13.7 31.2 3.6 0.9 0.7 3.9 0.6 56 27.1 66.9 4.6 1.2 0.5 1.4 0.2 w 302 63.4 5.6 1.3 0.6 1.6 0.3 1%

36.3 71.2 6.6 1.6 0.7 2.2 0.4 122

29.0 76.4 5.5 1.3 0.. 0.9 0.2 117

29.0 63.4 5.6 1.3 0.6 1.6 0.3 106

16.3 44.3 3.1 0.6 0.4 1.4 0.3 73

19.1 35.6 3.0 0.7 0.3 1.4 0.3 67

a.0 22.0 2.1 0.6 0.3 1.4 02 42

9.7 0.0 2.0 0.7 0.4 1.6 0.3 P

17.9 69.3 3.4 0.7 0.4 2.2 0.4 92

342 74.6 6.0 1.1 0.6 2.1 0.4 123

40.4 71.3 6.4 1.3 0.7 2.3 0.. 128

36.9 64.7 4.0 0.6 0.4 1.4 0.3 112

4.6 12.4 1.7 0.6 0.6 3.2 0.6 38

IO.6 19.6 32 1.0 0.6 2.0 0.3 51

47.6 102.0 12.0 2.5 2.0 7.5 1.1 165

34.0 62.0 9.6 2.3 1.7 7.0 1.0 126

26.4 62.9 0.6 2.3 1.7 7.6 1.1 114

36.3 72.2 10.0 2.9 1.6 5.2 0.B 133

46.0 69.0 6.4 1.6 0.6 2.6 0.5 164

422 61.6 7.6 1.4 0.6 2.9 0.6 142

262 58.2 5.7 1.3 0.6 2.7 0.4 102

372 58.2 7.2 1.2 0.7 3.1 0.5 (14

42.9 50.5 6.6 1.4 0.6 2.0 0.6 121

46.3 88.4 7.6 1.6 0.6 3.4 0.6 153

21.9 46.4 4.7 0.9 0.6 3.0 0.6 64

35.4 41.6 6.4 1.3 0.9 3.1 0.6 %

36.4 61.6 6.6 1.3 0.7 3.5 0.6 117

63.6 107.0 9.9 1.6 1.1 3.6 0.7 164

34.9 69.. 6.1 1.3 0.7 2.0 0.5 I,3

66.0

45.7

23.0

20.7

291.0

94.7

46.6

32.5

35.3

15.7

41.6

46.3

U.0 795 47.0

23.4

60.9

66.4

64.5

47.4

54.9

465

51.1

79.4

46.0

47.0

106.0

04.0

35.1

37.1

600.0

169.0

?I!.0

62.0

67.6

29.1

61.6

100.0

137.0

96.4

46.6

%.4

98.6

64.6

p8.3

112.0

91.0

99.0

161.0

67.0

95.4

11.0 1.6 1.3 4.6 0.6 162

6.9 1.4 0.0 4.2 0.7 1%

2.4 0.4 0.3 1.6 0.3 76

2.1 0.3 0.2 12 02 76

62.0 9.1 6.9 20.9 3.0 903

17.7 3.7 2.6 6.1 1.1 336

6.1 1.5 (2 3.5 0.6 192

6.3 1.3 1.0 4.6 0.6 126

9.6 1.9 1.6 5.4 0.9 1%

3.6 0.7 0.6 3.3 0.6 73

7.1 1.2 0.9 3.7 0.6 153

7.0 1.5 0.9 3.4 0.6 179

9.4 1.3 12 6.2 0.7 164

11.7 2.3 1.1 5.5 1.0 250

9.4 1.7 12 6.6 0.0 163

4.6 0.6 0.7 3.5 0.6 101

6.9 I.4 0.7 3.6 0.6 169

9.6 1.6 1.1 3.9 0.6 1%

6.9 1.6 1.0 3.6 0.6 172

9.8 1.4 1.2 6.1 0.6 160

10.0 1.7 1.1 4.2 0.6 202

11.5 2.2 1.6 5.6 0.6 167

10.9 2.1 1.5 6.0 0.0 169

16.6 2.7 1.6 6.6 1.1 266

7.9 1.4 0.6 2.7 0.4 1%

9.o 1.6 1.1 4.2 0.7 178

composition, and may potentially be used as provenance indicators for cratonic sedimentary sequences.

6. SYNTHESIS

The progressive evolution in the average composition of samples through time implies a first-order control on mudrock chemistry. There are two possible first-order controls on mudrock composition: ( 1) evolution of the composition of the exposed crystalline upper crust; and (2) progressive weathering of sedimentary material over a series of recycling events. Both processes have been active in the history of the Colorado Province.

( 1) Evolution of upper crustal composition in the Colorado Province is evident from Fig. 3, which shows that the earliest

continental crust in the Colorado Province had a high proportion of mafic and intermediate rocks, but that subsequent additions were overwhelmingly granitic (Reed et al., 1987; Bick- ford, 1988). The slope of the curve is a function of the rate of addition of granitic material to the crust, but not necessarily the rate at which that change would be evident in the sedimentary record. There is a variable lag time between the crystallisation of igneous rock and its exposure at the surface, and therefore the rate of increase of the granitic component of sedimentary source rocks is probably somewhat lower.


Table 4 Continued

sequence Sample R LOC’ La Ce Sm Eu Tb Yb LU XREE

Ap.YR.470

Ap.RD.514

H.MC.535

H.uC.535

s.c.545

H.Tr.55E.b

H.TT.566b

H.Tr.E&?

H.Tr.563

n-l-r.564

H.Tr.577b

H.PC.576b

H.PC.563b

KPC.566

I.LC.254

T.J.365

BA.PB.377

EA.MC.531

EA.MC.532

EA.MC.533

BA.PC.625

BA.PCS26

EA.PC.527

BA.PC.626

EA.PC.630

q A.PC.631

EA.PC.633

EA.PC.634

EA.lT.635

0A.PC.637

T.GC.636

T.GC.639

M.PR.320 D.FC.217

D.FC.224

D.TC225A

DAR239

D.FC.250

O.BY314

D.CB.337

o.cs.420

o.cs.421

3

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4

4

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57.7 119.0 9.5 1.5 1.0 3.9 0.6 221

40.3 91.4 9.5 1.6 1.3 4.5 0.7 179

15.0 37.6 7.2 1.2 0.7 3.1 0.5 90 IQ.6 42.0 4.7 0.9 0.7 2.6 0.4 96 41.6 63.0 7.9 1.1 0.6 2.9 0.5 (41

30.3 66.9 7.1 0.9 1.0 4.2 0.6 113

26.3 56.5 6.9 1.2 1.1 4.2 0.6 102

30.Q 56.6 7.0 9.5 1.0 3.5 0.5 106

35.1 66.0 12.3 2.4 1.2 3.0 0.6 125

16.6 95.9 33.6 7.8 7.4 6.7 1 .o 174

24.0 49.6 5.7 1.1 0.7 3.0 0.5 66

302 57.9 6.6 1.t 0.9 3.7 0.6 127

472 90.7 6.2 1.4 1.1 4.6 0.7 180

31.6 61.4 7.0 1.2 1.2 3.4 0.6 133

30.6 63.7 7.1 1.2 1.0 3.6 0.6 126

40

26

37

36

36

38

37

37

37

37

37

37

37

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3,

44.3 92.0 6.1 0.6 0.6 5.6 0.9 193

103.0 222.0 16.6 2.7 2.4 10.7 1.6 393

94.6 162.0 (9.2 1.7 1.2 6.0 0.9 339

90.3 196.0 9.6 1.3 0.6 6.6 0.9 349

71.9 (13.0 a.7 t.2 0.6 4.1 0.6 242

60.0 124.0 9.9 1.6 I.1 6.0 0.6 244

7j.6 136.0 9.7 1.3 1 .o 5.1 0.8 r.s7

70.1 (29.0 6.6 1.4 1.0 5.1 0.6 257

61.1 116.0 6.0 3.1 0.6 5.2 0.6 235

54.7 123.0 7.5 1.1 0.7 4.6 0.7 233

67.9 126.0 9.5 1.4 1.2 5.7 0.9 258

61.2 164.0 6.8 1.2 0.9 ..9 0.6 303

60.7 169.6 9.6 1.4 1.7 a.7 1.2 314

63.3 116.0 7.9 1.2 0.9 5.6 0.6 237

57.1 117.1 9.3 1.3 0.9 6.2 0.6 243

60.1 133.0 122 1.6 2.2 9.1 1.1 261

51.0 14j.o 10.6 1.6 1.4 6.6 1.2 281

60.0 163.0 15.3 2.5 1.a 6.5 9.0 305

69.0 131.0 9.6 1.3 1.0 6.5 0.9 256

46 33.7 62.4 6.. 1.0 0.6 2.6 0.4 159

42 40.4 73.0 6.6 1.1 1.0 4.3 0.7 174

42 39.9 71.4 7.0 3.1 0.6 3.2 0.6 171

43 33.6 60.1 5.2 0.6 0.4 2.1 0.3 161

44 31.5 60.3 5.7 1.0 0.7 2.6 0.4 151

42 32.6 67.6 7.1 1.0 0.7 3.0 0.6 150

45 26.5 67.2 5.2 0.6 0.5 2.6 0.4 145

46 39.7 76.5 6.7 1.1 0.7 3.1 0.5 179

46 61.j 121.0 10.0 1.6 1.2 5.4 0.6 252

46 36.9 73.6 7.1 I., 0.5 3.2 0.5 176

36.4 66.9 6.7

2319 .16D3 8470

1.0

2399

0.7 3.0 0.5 164

(2) Recycling of older sedimentary rocks was significant in the Colorado Province because, subsequent to the erogenic events which accompanied crustal accretion (Fig. 2), deformation in the Colorado Province was dominated by vertical movements, mainly arching and block faulting (Keith and Wilt, 1986; Baars et al., 1988; Sloss, 1988), which were rarely sufficient to completely strip the sedimentary cover and expose basement on a regional scale. Evidence that much of the sediment in each of the Colorado Province sedimentation intervals subsequent to RI was derived by recycling of older sedimentary sequences is provided by the increasing abundance of sedimentary clasts in conglomerates, and by the composition of associated sandstones (Cox, 1993; Cox and Lowe, 1995b). Colorado Province sandstones also record a progressive increase in compositional maturity, due to a com-

bination of protracted weathering of sedimentary detritus during multiple recycling episodes and decreasing average input of basement-derived detritus to the sedimentary system (Cox and Lowe, 1995b).

The major element chemistry of the Colorado Province mudrocks, which indicates a general increase in the proportion of clay minerals at the expense of nonclay silicate minerals, reflects increasing average compositional maturity of the sediment mass. In addition, the examination of the ICV

and the K20/A120r ratio demonstrate that early mudrocks contained little K-feldspar whereas later nonclay silicate additions to mudrocks were dominated by K-feldspar. Trace element distributions indicate that an increasing proportion of fractionated igneous material was being input to the sedimentary system over time. These observations are consistent with


0.4 J . . RO RI RZ R3 R4 R5

(b)

T, pq . 1 Ia0 p = 0.0200 14c I I:

. . . . . . RO RI IU R3 R4 R5

FIG. 12. Graphs of REE data as a function of sedimentation interval. For explanation of p statistic, see caption to Fig. 7. Dots represent data points; circles represent group medians. Bars represent 94-98% confidence intervals about the averages. Subscript n denotes data normalised to chondrites: normalisation factors are from Wakita et al. (1971). Eu* is calculated by extrapolating between Sm and Th on a graph of relative abundance vs. atomic number, and applying the equation for a geometric progression:

Eu*=Tb. rsmlrt, ‘. ( )

Two data points are not shown on the graphs for reasons of scale: (a) One value in R2: ZRJZE = 900 ppm; (b) One value in R2: ZREE. = 485.

the tectonic evolution of the Colorado Province from a series of accreted arc terranes composed of igneous and metamorphic rocks with a wide range of compositions, including a substantial proportion of mafic material, to a stable craton with a high proportion of granitic upper crustal material and a well-developed sedimentary mantle.

Acknowledgmenrs-This work formed part of Rdnadh Cox’s Ph.D. research at Stanford University. Many thanks to Lew Ashwal, 1. Veizer, K. A. Eriksson, C. Frost, and S. M. McLennan for providing helpful reviews of this paper, to Konnie Krauskopf, Yeshu Kolodny, Liz Eide, and Mark Brandriss for extensive discussion of some of the problems, and to David Gonzales, who collected some of the samples. Funding for this work came from several sources, including the Dean

and Dorothea McGee Foundation at Stanford, GSA Penrose grants 4 174-89 and 461 l-9 I to R6nadh Cox and NSF grants EAR 88-92618 and 89-04830-A I to Don Lowe.

Edirorial handling: S. M. McLennan

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